Vapor generator



VAPOR GENERATOR 4 Sheets-Sheet 4 Filed July 27, 1961 I- LOAD FIG. IO

INVENTORS. ELNO M. POWELL VIRGINIUS Z. CARACRISTI BYZ'AIM A4 XJQ.

ATTORNEY United States Patent Ofl ice Patented June 2, 1964 Filed July 27, 1961, Ser. No. 127,185 20 Claims. (Cl. 122-440) This invention relates generally to the vapor generating art and has specific relation to an improved method and apparatus for generating vapor in accordance with a variable load or demand.

The invention is concerned with a vapor generator of the forced through-flow type with the generator beingoperat ed on the reheat cycle and being provided with a pair of furnaces which are independently fired. In accordance with the invention the vapor generator is arranged in such a manner that bias firing may be utilized as one of the controlactions for controlling reheat temperature with varying load, with bias firing being the firing of the twofurnaces at different rates.

In order to employ bias firing in a once-through flow type vapor generator, such as vapor generators operating at supercritical pressure, and at the same time maintain a relatively simple control arrangement or system for the primary vapor that is forced through the through-flow circuit, it is necessary to arrange the various heat exchange sections which make up the through-flow circuit in such a manner that the through-flow is not divided into a number of streams so located as to be heated to substantially different temperatures which must then be mixed to provide a uniform temperature of the fluid. To

insure that this does not occur, it isnecessary that the various heat exchange sections which are connected in series flow relation and which make up the through-flow circuit be subjected to the heating influence of only one furnace or one gas stream issuing from a furnace, with the exception of the economizer circuits. -The tempera tures of both the heating gas and the working medium are sufficiently .low in the economizer section so that any temperature differences which may result because of bias firing will be of no consequence.

In accordance with theinvention there is Provided a vapor generator which is comprised of two units each of which includes a furnace and a gas pass extending there from. The generator is operated on the reheat cycle and in one embodiment the single reheat cycle is employed while in another embodiment the double reheat cycle is used. The generator is of the forced through-flow type and is preferably operated at supercritical pressures and the through-flow circuit includes tubular heat exchange sections-disposed on the walls of each of the furnaces with these sections being connected in series flow relation. After passing through these furnace sections the through-flow is conveyed serially through several other heat exchange sections which also form part of the through-flow system and each of these sections is subject- .ed to the heating influence of only one of the units with some of the sections being associated with one unit and others being associated with the other unit.

In the embodiment emp'loyinga single reheat the reheater is arranged so that it receives a major portion of its heat from one of the units being preferably in the gas pass of that unit. The surface of the generator including the 'heat exchange sections in the throughdlow circuit andthe reheater are arranged so that the desired primary fluid temperature and the desired reheat temperature are provided at a predetermined or maximum load on the unit. As the load isdecreased from this predetermined or maximum load the total fuel fired to the unit and the flow through the through-flow circuit are controlled in a manner so that the desired pressure and termperature of the primary fluid egressing from the throughi'low circuit are maintained and additional control action is provided to maintain the reheat temperature at its desired value. This additional control action includes adjustably proportioning the fuel between the two furnaces; increasing the relative proportion supplied to the furnace of the unit responsible for'reheating the Working medium. In addition to this bias firing a further control action may be provided such as adjusting the zone of combustion in thefurnace or introducing recirculated combustion gases thereinto to provide the desired range of control for the reheat temperature.

in the embodiment of the invention employing the double reheat cycle the bias firing control is utilized to compensate for the tendency of the two reheat temperatures to vary with respect to each other with varying load. In a vapor generator of the type described and operating on the double reheat cycle the generator will be designed so that at a predetermined or the maximum load each of the reheat temperatures in addition to the primary temperature will be at its desired value. In addition to the characteristic that the reheat temperatures tend to decrease with decreasing load, the reheat temperatures will vary relative to each other with varying load. Depending upon the particular unit and the entire operating cycle of the power plant of which the vapor generator forms a part, the high pressure reheat vapor may have a tendency to either increase or decrease relative to the low pressure .reheat vapor as the load is decreased from the predetermined or maximum value. The bias firing control inaccordance with the invention is utilized to overcome this tendency, with an additional control action such as varying the zone of combustion in each of the furnaces or recirculating gases to each of the furnaces being utilized to maintain the two reheat temperatures at their desired value throughout the range of operation of the vapor generator.

Accordingly it'is an object of this invention to provide an improved vapor generator of the forced through-flow type having a plurality of furnaces with gas passes extending therefrom and which operates on the reheat cycle.

A further object of the invention is to provide such an improved vapor generator wherein bias firings of the units may be utilized to provide a control action for reheat purposes without having an unduly complicated system and control for the primary fluid and circuit.

A still further object is to provide such an improved vapor generator operating on the single reheat cycle and employing bias firing to regulate at least in part the temperature of the reheat ,vapor with varying load.

Stillanother object of the invention is to provide such an improved vapor generator operating on the double reheat cycle and employing bias firing to overcome the tendency of two reheat temperatures to vary'with respect to each other.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization insuch a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description 'of an illustrative embodiment, said embodiment being FIG. 2 represents a transverse sectional view through the furnaces of the generator represented by FIG. 1 and discloses the configuration of the furnaces and shows that they are separate furnaces in side-by-side relation;

FIG. 3 is a fragmentary elevational view of a prtion of one of the walls of the furnaces in FIG. 2 showing that the tubes are vertically arranged in side-by-side relation and are welded together;

FIG. 4 is a sectional view taken generally along line 4-4 of FIG. 3;

FIG. 5 is a diagrammatic view in the nature of a vertical section from front to back of one of the units of FIG. 1 and shows the general configuration that the unit have the general disposition of the heating surface in the units;

FIG. 6 is a view similar to that of FIG. 5 but showing a modified control arrangement with FIG. 5 illustrating a tilting burner facility while FIG. 6 provides a gas recirculation type of control;

FIG. 7 is a diagrammatic representation in the same general nature as that of FIG. 1 but representing a vapor generator on the single reheat rather than double reheat cycle;

FIG. 8 is a set of curves illustrating a characteristic operation of the double reheat generator of FIG. 1 and shows reheat temperature plotted against load and illustrates how the reheat temperatures will tend to decrease with load, how the bias firing action overcomes the tendency of the reheat temperature to vary with respect to each other and how the other control action maintain the reheat temperature generally constant; and

FIG. 9 is a set of curves of the general nature of that of FIG. 8 but representing operating characteristics obtained with the single reheat unit of FIG. 7.

FIG. 10 is a graphic representation of the control that can be achieved with the invention in a unit operating on the double reheat cycle.

Referring now to the drawings, where like reference characters are used throughout to designate like elements the illustrative embodiment of the invention illustrated in FIG. 1 comprises a vapor generator that is made up of two units identified as A and B with each of the units including a furnace and a gas pass extending therefrom and as represented the furnace of unit A is identified as 10 with its associated gas pass being identified as 12 while the similar elements of unit B are identified as 14 and 16, respectively. The vapor generator is preferably operated at supercritical pressure and is provided with a through-flow circuit through which primary fluid or Working medium is forced by means of feed pump 18.

In the FIG. 1 illustration the working medium leaves the pump 18 at supercritical pressure, passes through the flow control valve 20 and then through conduits 22 and 24 to the economizer heat exchange sections 26 which form part of the through-flow circuit. As illustratively disclosed these economizer heat exchange sections are in parallel relation and after traversing these sections the through-flow is conveyed via conduits 28 and 30 to the mixing vessel 32. From this mixing vessel the flow is conveyed by conduit 34 to suitable headers at the lower end of furnace 10 and with which the vertically extending tubes that line the walls of the furnace communicates with these tubes being identified as 36 in FIGS. 2, 3, and 4. These tubes 36, which traverse the furnace only once, are in parallel flow relation and the fluid is directed upwardly therethrough from the conduit 34 and into suitable headers at the upper end of the furnace with which the tubes are also connected. After leaving the furnace wall tubes of furnace 10 the through-flow is directed via conduit 38 to the lower end of furnace 14 which has similarly arranged tubes 36 with the throughflow being similarly directed upwardly through these tubes. After traversing the furnace wall tubes of furnace 14 the through-flow is in serially conveyed through a first fluid heater identified as 40, a second fluid heater identified as 42 and finishing fluid heater identified as 44, with the fluid heater 40 being in the gas pass 16 while fluid heaters 42 and 44 are in the gas pass 12. The throughflow is conveyed from the furnace wall tubes 36 of furnace 14 to fluid heater 40 through conduit 46, from the heater 40 to the heater 42 via conduit 48 and from the heater 42 to the finishing heater 44 via the plurality of conduits 50. After traversing the finishing fluid heater 44 the working medium is conveyed through the conduits 52 to the conduit 54 which is connected with the high pressure stage of turbine 56.

The vapor generator of FIG. 1 operates on the double reheat cycle with high pressure reheater 58 being located in the gas pass 16 and the low pressure reheater 60 being located in the gas pass 12. After passing through the high pressure stage of turbine 56 the vapor is conveyed via conduit 62 to the high pressure reheater, returning to the turbine after being'suitably reheated to its desired temperature via conduit 64. The reheated vapor is then expanded through a second stage of the turbine and is thereafter conveyed to the low pressure reheater 60 through conduit 66, returning from this reheater after being suitably reheated via conduit 57.

Each of the fluid heaters 40, 42, and 44 as well as the reheaters 58 and 60 and the economizers 26 are comprised of a substantial number of tubular elements that are bent generally in a sinuous fashion so as to form a tube bundle that extends generally the full width of the gas pass in which it is disposed with the tubes of the bundle being in parallel flow relation with respect to the flow of the recirculating medium. It is the general practice, as illustrated in the diagrammatic representation of FIG. 1, to have the interconnections between the second fluid heater 42 and the finishing fluid heater 44 as well as the outlet connections of fluid heater 44 comprise a plurality of parallel flow connections and with desuperheaters being employed so as to provide a control for variations in the final temperature leaving the through-flow circuit and delivered to the turbine which are caused by transient conditions, or these desuperheaters may be employed in order to provide a temporary anticipatory control action.

As illustrated each of the conduits 50 is provided with a desuperheater 68. The desuperheaters in each of these conduits 50 are controlled in response to a temperature sensing device 70 in the corresponding conduit 52. Each of these temperature sensing devices is connected with a suitable control device 72 which in turn regulates the desuperheater through the suitable actuator 74. It will be understood that the control as provided by the desuperheaters is only a temporary type of control with the main control for the temperature and pressure of the primary fluid delivered to the turbine being effective to regulate the total fuel fired and the feed supply to the throughfiow circuit in a manner to maintain these parameters generally constant throughout the desired load range over which the vapor generator is to operate.

A very schematic and simplified representation of a control is set out in FIG. 1 which includes a load responsive device 76 and a temperature responsive device 78. These devices are connected with a suitable control mech anism 80 which is effective to control the ratio of the feed supplied to through-flow circuit and the firing rate of the vapor generator with these controls being represented by the regulators 82 and 84 which are regulated by the control device 80 with the regulator 82 controlling the feed control valve 20 and the regulator 84 controlling the fuel and air control 86. This latter valve is illustratively disclosed as mounted in the conduit 88 which connects with the branch conduits 90 and 92 that lead to the burners 94 of the furnaces 14 and 10, respectively.

The vapor generator is designed so that at its maximum load of operation, or the so-called design load, the temperature of the primary fluid supplied to turbine 56 through conduit 54 as well as the temperature of the high pressure reheat and low pressure reheat vapor supplied to the turbine may be at their desired value. As the load on the vapor generator decreases from this maximum or design value, the high pressure and the low pressure reheat temperatures tend to decrease and additionally tend to vary with respect to each other. Accordingly, if no control action were provided to counteract these effects,

these reheat temperatures would steadily become lower and would become substantially different from each other as the load falls. This situation prevails even though the primaryfiuid is maintained at its desired pressure and temperature by suitably controlling the feed supply and the total firing rate of the vapor generator.

The reheat temperatures are maintained generally constant throughout the operating load range of the vapor generator by a plurality of control actions. Bias firing is used to overcome the tendency of the high pressure and low pressure reheat temperatures to vary with respect to each other while an additional control action is utilized to maintain the temperatures of both of the reheats at their desired value. In the FIG. 1 illustration the high pressure reheat temperature is sensed at 96 providing an indication received in controller 98 while the low pressure temperature is sensed at 100 providing an indication also received in the controller 98. From this controller signals are received in the regulating devices 102 and 104 which control valves 106 and 108, respectively, to adjustably proportion the fuel between the two furnaces 14 and 10.

The controller 98 is also connected with the load responsive control device 76 and the control action that is provided to regulate the valves 1% and 108 is such as to proportion the fuel and air delivered to the two furnaces in a manner to overcome the tendency of. the two reheat temperatures to vary with respect to each other with varying load.

FIG. 8 illustrates the characteristic of the reheat temperatures with a decrease in load with the curve identified as 110 showing how the low pressure reheat temperature would vary in one particular unit and cycle if there were no control actions supplied and curve. 112 showing how the high pressure reheat temperature would vary. The curve identified as 114 is the temperature curve that would be produced if only the bias firing (adjustably proportioning the fuel and air. between the two furnaces) control were employed with the bias firing being effective to equalize the two reheat temperatures. In addition to the bias firing control for the reheat, an additional control to bring each of the reheat temperatures up to its desired value is provided. In the arrangement illustrated in FIGS. 1 and 2 tilting tangential burners are employed with each of the furnaces with these burners being effective to introduce the fuel and air tangent to a centrally located vertically disposed imaginary cylinder in each furnace with the burners being tiltable so as to adjust the zone of combustion vertically Within the furnaces toward and away from the furnace outlet. The control is such that as the load is decreased the zone of combustion is moved toward the furnace outlet increasing the heat content of the gases traversing the reheater' heat exchangers and accordingly maintaining the reheat temperaturesat their desired value with the curve 116 of FIG. 8'illustrating the reheat temperatures as thus controlled with varying load. The control of the tilting burners is provided through controller 98 and supplemental control devices 118 and 120 which are eifective to adjust the burners 94 for each of the furnaces independently and provide the necessary control action.

A very practical way of combining the bias firing and the tilting burner control (or other control such as gas recirculation) is'to utilize the tilting burner control to its full extent in the furnace Where tilting burner control gives the least control range and then utilize both bias firing and the remaining tilting burner control in the other furnace to obtain a further extension of the control range. The result of this type 'of operation is disclosed in FIG. 10. The generally straight line identified as 111 represents the temperature at which it is desired to control the two reheats. As the load is decreased from the design or percent value, tilting burner control is utilized in each furnace to maintain the two reheats at their desired value so that their temperature lies along this 111. At the point 113 on this line the tilting burner control has reached the limit of its ability to control the temperature of one of the reheats, for example, the high pressure reheat. Accordingly, if no other control action were initiated the high pressure reheat temperature from the point 113 would follow the curve as the load is further decreased. However, with respect to the other reheat temperature (the low pressure reheat in the example chosen) tilting burner control would be effective to maintain the temperature of this reheat at its desired value to the point identified as 117. Accordingly, at the point 113 bias firing may be utilized to extend the control range over which the two reheat temperatures may be retained at their designed value. At this point 113 bias firing may be utilized to increase the proportion of total fuel fired in furnace 14 relative to that in furnace 10 to bring the high pressure reheat temperature up to its desired value. Since at point 113 the tilting burner control is still effective to maintain the low pressure reheat at its desired value the fact that the firing rate in furnace 10 is proportionally decreased as the load is decreased beyond point 113 will be compensated for by means of the tilting burner control. The bias firing is thus elfective to overcome the tendency of the two reheat temperatures to develop different temperatures and the control range, by means of the combination of bias firing and tilting burner, may be extended to the point identified as 114. This is the limit of the control of the two reheat temperatures that can be obtained with this combination of control actions with the reheat temperatures decreasing as represented by curves 121 as the load decreases beyond that represented by the point 119. By combining the tilting burner and bias firing controls in this manner, a relatively simple arrangement may be utilized to extend the control range over that which may be had with tilting burners, per se.

FIG. 5 illustrates the general layout or configuration of one of the units of the steam generator of FIG. 1 with this FIG. 5 disclosing unit B and showing that the furnace is vertically disposed and that the gas pass extends from the upper end of the furnace downwardly with the various heat exchange sections being disposed so that to be traversed by the gas stream.

The detailed construction of the tilting tangential burn ers and their operation to regulate the heat content of the gases may be such as shown and described in US. Patent No. 2,363,875 granted November 28, 1944, to H. Kreisinger and V. Z. Caracristi.

In lieu of employing this tilting tangential type of control action, the gas regulation type of control may be utilized with FIG. 6 diagrammatically representing one of the units employing this type of control. In this type of control, combustion gases which have traversed the I reheat heat exchange portions are reintroduced into the furnace in such a manner as to increase the heat content of the gases which traverse the convection heated reheater.

In the FIG. 6 illustration the furnace is fired by the front wall burners 12:2 and gases recirculated to the unit through the duct 124 and the recirculation fan 126, with adjustable damper 128 being controlled through the supplemental control 118 so as to regulate the recirculation of gases to maintain the reheat temperature at its desired value. It will be understood that when recirculation type of control is employed each of the units will be provided with an arrangement such as disclosed in FIG. 6.

The turbine 56 drives the electric generator 136 which supplies a suitable load and from which the load responsive device 76 receives its impulse. The efiluent from turbine 56 is received in condenser 138 Where the vapor is condensed with condensate pump 140 forcing the condensate through fluid heater 142 and deaerator 144 to the feed pump 18.

. The supercritical pressure through-flow circuit is illustratively disclosed is provided with a recirculating circuit that is eflective to recirculate working medium through the furnace wall tubes. In the forced through-flow type of vapor generator and particularly when operating at supercritical pressure a problem arises in providing adequate cooling for the furnace wall tubes as the load on the unit decreases. This is so because the flow of the working medium decreases with the load and accordingly the velocity through the furnace wall tubes becomes less. By providing a recirculation of effluent through the furnace wall tubes this problem can be overcome and moreover a more desirable type of furnace wall construction may be employed with larger tubes being thereby permitted and with a construction which employs parallel flow furnace wall tubes also being permitted. The recirculation of fluid is effected by means of the recirculating conduit 146 which is connected at its inlet with the conduit 46 and has disposed therein the recirculating pump 148 and check valve 150 and which connects with the mixing vessel 32. The recirculating circuit and particularly the recirculating pump arrangement, which may be placed either in the recirculating conduit 146 or in the conduit 34 is preferably of the centrifugal type operating at a constant speed and is floating on the through-flow circuit so as to automatically provide the necessary flow to insure that an adequate velocity is had through the furnace Wall tubes 36 during the entire operating load range of the vapor generator with this pump arrangement being described in detail in the application of Willburt W. Schroedter entitled Recirculating System for Steam Generator (application Serial No. 127,395) now Patent No. 3,135,252 of June 2, 1964 filed concurrently herewith and assigned to the same assignee.

The furnace wall construction, as previously mentioned, is comprised of vertically extending parallel flow tubes which traverse the furnace only once with these tubes being welded together as shown in FIGS. 3 and 4 so as to form a generally imperforate inner wall surface for the furnace. The recirculating arrangement superimposed on the through-flow circuit of the supercritical vapor generator enables the use of this type of wall construction without developing excessive stresses in the welded tube inner wall surface. The recirculation reduces the temperature variation between the inlet and outlet of these tubes which in turn has the effect of providing a more uniform temperature distribution transversely of the furnace walls. A more uniform temperature is also provided by being able to use tubes which are of larger diameter than would be permitted without the recirculating system.

By having each of the heat exchange sections in the through-flow circuit that are downstream of the economizer section with regard to the flow of the working medium being subjected to the heating elfect'of only one of the units, the bias firing control does not result in providing two streams of the primary fluid at substantially different temperatures which would be the case if one or more of the heat exchange sections were subjected to the heating effect of both of the units. Furthermore by arranging the finishing fluid heater so that it is located entirely within one of the units the number and complexity of the desuperheating control is substantially decreased and the outlet of the finishing fluid heater may be more advantageously located with respect to the delivery of the primary fluid to the turbine.

In lieu of employing the double reheat cycle the vapor generator may be operated on the single reheat cycle with FIG. 7 providing a diagrammatic representation of such a system. In this FIG. 7 representation the primary circuit or through-flow circuit is the same as that in the FIG. 1 representation with the single reheater identified as 152 receiving working medium from turbine 56' through conduit 154 and delivering the reheated vapor to the turbine through conduit 156.

In the FIG. 7 arrangement the bias firing of the two furnaces is effective to control the reheat temperature with this bias firing acting as a supplementary control to either tilting burner or gas recirculation control.

FIG. 9 illustrates the effect of the control actions that are obtained with this modified system. In this figure, the curve 158 represents the temperature characteristic of the reheat vapor that would prevail if no control action were provided to maintain the reheat temperature generally constant throughout the operating load range. The curve 160 represents the characteristic temperature that may prevail if bias firing is employed alone and the curve 162 represents the temperature characteristic of the reheat steam that may prevail with the combination of bias firing and tilting burner control. By means of bias firing in the FIG. 9 arrangement the reheat temperature may be controlled for a predetermined extent of the load range. In order to provide a satisfactory control for the reheat temperature over a wider load range the bias firing control may be supplemented by adjusting the zone of combustionlongitudinally within the furnace as explained hereinbefore or by means of reintroducing recirculated gases into the furnace as explained hereinbefore.

In both the double reheat system and the single reheat system the bias firing control is effective to increase the range of control over which the reheat vapor temperature may be regulated and maintained at its desired value.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be madetherein Without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What is claimed is:

1. In a once-through flow vapor generator operating at supercritical pressure and on the reheat cycle and comprising a plurality of units each of which has a furnace with each furnace having a gas pass extending therefrom, said gas passes being in parallel flow relation, and each of which furnaces has suitable firing means associated therewith so that fuel is introduced and burned therein producing a combustion gas stream, the improved method comprising directing the entire primary fluid of the generator first through one furnace, and thereafter through the other furnace, and then directing said fluid in serial heat exchange relation with the gas stream produced in each of the units thereby heating the primary fluid to its desired temperature, directing said fluid in heat exchange relation primarily with the gases in one of said units to reheat said fluid after utilizing a portion of its energy and at a location in said one gas stream where the heat content thereof is such that the reheat temperature tends to decrease with decrease in load, adjustably proportioning the firing of the several units with varying load by increasing the relative proportion of said one unit with decreasing load.

2. The method of producing vapor comprising burning fuel at a first zone and creating a combustion gas stream and burning fuel at a second zone and creating another combustion gas stream, forcing generally the entire primary fluid in a continuous path first through the first and 7 then through the second zone and thereafter in serial heat exchange relation with the two gas streams thereby imparting heat to the fluid and heating it to its desired temperature, directing said fluid in heat exchange relation primarily with the gases one of said gas streams to reheat said fluid after utilizing a portion of its energy and at a location in said one gas stream where the heat content thereof is such that the reheat temperature tends to de crease with decrease in load, and of the total fuel burned at the two zones increasing the proportion burned in the zone creating said one gas stream as the load decreases.

3. The method of generating vapor comprising burning with the two combustion gas streams with the working medium being heated to its desired value upon traversal of said circuit, supplementing the through-flow in said confined streams by recirculating fluid in the portion of said circuit comprised of said confined streams, utilizing a portion of the energy of the thus heated working medium and thereafter reheating the same by passing it in heat exchange relation primarily with one of said gas streams and at a location in said stream where the heat content thereof is such that the reheat temperature tends to decrease with decrease in load providing regulating functions including regulating the total fuel fired to regulate the heat absorption of the continuous circuit to maintain the temperature of the working fluid after traversal of said circuit generally at its desired value throughout a predetermined load range and adjustably proportioning the fuel burned at said two zones with the proportion burned in the zonecreating said one combustion gas stream increasing with decreasing load.

4, A forced through-flow vapor generator comprising a pair of units each of which includes a furnace with a gas pass extending therefrom, separate means for firing said furnaces, a through-flow circuit through which the primary fluid is conveyed, said circuit including an economizer section in each of said gas passes, a heat exchange section in each of said furnaces with these sections being connected in series flow, means operative to force genorally the entire primary fluid of the generator first through one of these heat exchange sections and then through the other and additional heat exchange sections downstream of the furnace sections with regard to the flow of the primary fluid with each of said additional sections being subject to the heating influence of a single unit and connected in series flow relation with regard to flow through said circuit, reheater means receiving the working fluid after a portion of its energy has been utilized and disposed to receive a major portion of its heat from one of said units and at a location where the heat input thereto is such that the reheat temperature tends to decrease with decrease in load, means operative to con trol the total firing of the two units so as to maintain the final temperature of the primary fluid generally constant throughout the operating load range and means for adjustably proportioning the firing of the two furnaces to increase the firing of the furnace of said one unit relative to that of the other unit with decreasing load.

5. In a vapor generator operating on the single reheat cycle and including a pair of units each of which in- V cludes a furnace having an outlet, and a gas pass extending therefrom with the furnaces having separate means for firing the same, a through-flow. circuit through which generally the entire primary fluid of the generator is forced and which contains a plurality of heat exchange sections in series flow relation including a section in each of the furnaces connected so that generally the entire primary fluid of the unit traverses firstthe section in one I furnace and thereafter the section in the other furnace and additional sections downstream of the furnace sec- .tions with regard to flow of the primary fluid, each of said perature tends to decrease with decrease in load, the imprimary fluid and the reheat fluid in sufficient quantity to provide the desired final primary and reheat temperatures, providing regulatory functions including regulating the total fuel supplied to the two units to maintain the primary fluid temperature at its desired value throughout the operating load range and increasing the proportion of fuel delivered to said one unit relative to that delivered to the other unit as the load decreases thereby compensating at least in part for the tendency of the reheat temperature to fall with decreasing load.

6. The improved method of claim 5 wherein the furnace walls of said one unit are lined with heat exchange tubes and the reheater receives a substantial portion of its heat by convection with the reheater being located such that adjustment of the zone of combustion toward the furnace outlet increases the heat absorption of the reheater comprising the steps of adjusting the zone of combustion in the furnace toward the furnace outlet with decreasing load and regulating the proportioning of the fuel between the furnaces of the two units and the adjustment of the zone of firing so as to maintain the reheat temperature at its desired value throughout a substantial load range.

7. The improved method of claim 5 wherein the furnace of said one'unit has its walls lined with heat exchange tubes and the reheater receives a substantial portion of its heat by convection including the steps of recirculating combustion gases after traversal of thereheater back to the furnace in a manner to increase the heating effect of the gases egressing from the furnace and regulating the proportioning of the fuel between the furnaces of the two units and the recirculation of combustion gases so as to maintain the reheat temperature at its desired value throughout the operating load range of the vapor generator.

8. A vapor generator operating on the single reheat cycle comprising in combination a pair of units each of which includes a furnace with a gas pass extending therefrom, separate means for firing said furnaces, a throughflow circuit through which generally the entire primary fluid of the generator is forced at supercritical pressure, means for thus forcing said fluid through said circuit, said circuit including a heat exchange section comprised of tubes on the walls of one of saidfurnaces, another heat exchange section comprised of tubes on the walls of the other furnace, these heat exchange sections being in series flow relation such that generally the entire primary fluid of the generator first passes through one and then the other thereof, means superimposed on the throughflow circuit operative'to recirculate fluid through said furnace heat exchange sections, additional heat exchange sections in series flow relation and downstream of the furnace sections with regard to flow of the primary fluid with each of said additional sections being subject to the heating influence of a single unit, reheater means receiving the working fluid after a portion of its energy has been utilized and disposed to receive a major portion of its heat from one of said units and at a location where the heat input thereto is such that the reheat temperature tends to decrease with decrease in load, means operative to control the total firing of the two units so as to maintain the final temperature of the primary fluid generally constant throughout a predetermined load range and means for maintaining the reheat temperature generally constant throughout said load range including means independent of the 'firing rate operative to control, within limits, the heating effect of the combustion gases egressing from the furnace of'said one unit and means to adjustably proportion the firing of the two units, with the heating elfect and the proportion of fuel fired in said one furnace being increased with decreasing load.

9. The method of generating vapor comprising burning fuel at a first zone and creating a stream of combustion gases and burning fuel at a second zone and creating another stream of combustion gases, forcing the primary fluid through a continuous circuit first through said one zone and then through said other zone and thereafter in series heat exchange relation with the two combustion gas streams with the primary fluid being heated to its desired value upon traversal of said circuit, expanding said primary fluid at a first stage, reheating the working fluid a first time by passing it in heat exchange relation primarily with one of said gas streams, expanding the reheated fluid at a second stage, and reheating the working fluid a second time by passing it in heat exchange relation primarily with the other gas stream, regulating the total fuel fired so as to maintain the final primary temperature at its desired value throughout the load range, said first and second reheating being at locations where the heat content of the gas stream is such that as the load decreases from a desired maximum the temperature of the first and the second reheat tend to vary with relation to each other, and proportioning the relative firing of the two units so as to compensate for this tendency.

10. A vapor generator operating at supercritical pressure and on the double reheat cycle comprising a pair of units each of which has a furnace with an outlet and a gas pass, means for independently firing said furnaces, a through-flow circuit through which generally the entire primary fluid flows and comprising economizer heat exchange sections, a heat exchange section in each furnace connected in series flow relation with respect to the flow of the through flow so that generally the entire through flow passes through one of these furnace heat exchange sec tions and then through the other, and still further heat exchange sections in series flow relation with respect to flow of the through flow and each of which is subjected to the heating influence of a single of said units, a high pressure and a low pressure reheater each of which is subjected primarily to the heating influence on a different one of said units, means independent of the firing rate effective to vary the heat content of the gases egressing from each of said furnaces, said reheatersbeing disposed to be subjected to these gases, means effective to adjustably proportion the total firing of the generator between each of said furnaces, and means operative to regulate said proportioning means and said means to vary the heat content of the gases to regulate the reheat temperatures.

11. The organization of claim wherein each of the furnaces has its walls lined with heat exchange tubes with the tubes lining the walls of each furnace forming separate heat exchange sections which are part of the through-flow circuit and wherein the means independent of the firing rate for varying the heat content of the gases egressing from each furnace includes means to adjust the zone of combustion in each furnace towards and away from the furnace outlet.

12. The organization of claim 10 wherein each of the furnaces has the inner surface of its walls lined with heat exchange tubes with the tubes on each furnace comprising separate heat exchange sections connected in the through-flow circuit of the vapor generator and wherein the means independent of the firing rate effective to vary the heat content of the gases egressing from each of the furnaces includes means for recirculating combustion gases which have traversed the respective reheaters back to the furnaces associated with said respective reheaters.

13. In the operation of a forced through-flow vapor generator on the double reheat cycle the method comprising burning fuel in a first zone and creating a stream of combustion gases, burning fuel in a second zone creating a second stream of combustion gases, forcing a working medium through a continuous path where it is heated to its desired temperature and including directing the medium in heat exchange relation first with the burning fuel in one of said zones and then with the burning fuel in the other of said zones, thereafter directing the medium in series heat exchange relation with the two combustion gas streams and thereby heating the medium to its desired value, conveying the effluent from said continuous path to a point of use and utilizing a portion of the energy in said medium, thereafter reheating said medium by passing it in heat exchange relation primarily with the combustion gases produced in one of said zones, conveying this related medium to a point of use and again utilizing a portion thereof, thereafter reheating said medium by passing the same in heat exchange relation primarily with the combustion gas stream produced in the other of said zones, adjustably proportioning the total fuel that is fired between the two zones and varying the heat content of the gases egressing from each of said zones independent of the firing rate thereof to maintain the two reheat temperatures at their desired value with varying load.

14. A forced through-flow vapor generator operating at supercritical pressure and including a through-flow circuit through which the working medium is forced at supercritical pressure, a pair of furnaces into each of which fuel is introduced and burned to produce a combustion gas stream, each furnace having a gas pass extending therefrom, tubular members disposed on the walls of said furnaces, these tubes being connected into and forming a part of the through-flow circuit with the tubes in different furnaces being in series flow relation so that the through-flow passes first through said tubular members of one furnace and then through the tubular members of the other furnace, said through-flow circuit including additional heat exchange sections in series flow relation and downstream of the furnace tubes with regard to flow of the working medium with each such section being disposed in the gas stream produced by the burning fuel in a single of said furnaces, means superimposed on the through-flow circuit operative to increase the flow through the furnace wall tubes over and above that of the throughflow, a low pressure reheater having at least a predominant portion in one of said gas passes and a high pressure reheater having at least a predominant portion in the other of said gas passes and means operative to adjustably proportion the total fuel between the two furnaces so as to compensate for any tendency of the reheat temperatures to vary with respect to each other with varying load.

15. A supercritical vapor generator comprising a first unit having a furnace and a gas pass and a second unit having a furnace and a gas pass, means for firing each furnace with fuel, a through-flow circuit including economizer heating sections at the downstream end of the gas passes, furnace tubes in each of the furnaces and connected in series flow relation and additional of heat exchange means connected in series flow relation and subjected to the combustion gases from only one furnace, said furnace tubes and heat exchange means being connected into and forming part of the through flow circuit such that generally the entire through flow of the generator flows through said furnace tubes of one furnace, thereafter through the furnace tubes of the other furnace and then through said additional heat exchange means, a high pressure reheater in one of said. gas passes and a low pressure reheater in the other gas pass and at locations where the heat input is such that as the load decreases below a predetermined value the high pressure and low pressure reheat temperatures tend to vary with respect to each other and means operative to adjustably proportion the total fuel between the two furnaces so was to overcome this tendency.

16. In a once-through flow vapor generator having a pair of separate units each with its own furnace and gas pass and having a through-flow circuit which includes heating sections in each furnace and connected in series relative to the through flow such that generally the entire primary fluid passes through the heat exchange section in one furnace and then through the heat exchange section in the other furnace, and additional sections downstream of the furnace heating sections with regard to the through-flow with all of said additional sections being in series relation and with each such additional section beproviding a first reheat of the fluid by subjecting the same primarily to the heating influence of one of said units,

' providing a second reheat of the fluid by subjecting it primarily to the heating influence of the other of said units, this reheating being accomplished such that said generator has the characteristic that as the load changes the first and second reheat temperatures tend to change relative to each other, adjustably proportioning the total fuel between the two furnaces to overcome this tendency and varying the heat content of the gases egressing from each of the furnaces While maintaining the firing rates thereof at a desired value to maintain the reheat temperatures constant throughout said predetermined load range.

17. A once-through flow vapor generator operating on the reheat cycle and including a pair of units each of which has a furnace and a gas pass extending therefrom; means firing each of the furnaces, the through-flow circuit including a plufality of heat exchange sections connected in series flow relation with regard to the through-low so that generally the entire through-flow of the generator passes therethrough and each of which is associated with and receives heat from only one of said units, the walls of each of the furnaces being lined with heat exchange tubes with the'wall tubes of each furnace collectively forming oneof said heat exchange sections and with others of said heat exchange sections being comprised of tubular elements disposed in the gas stream issuing from the respective furnaces and with at least some ofthese heat exchange sections being downstream with regard to the through-flow of the furnace walltube heat exchange sections and with each of these downstream heat exchange sections being subjected to the heating effect of only one of the units, reheater means receiving a major portion of its heat from one of said units and at a location where the heat input thereto is such that the reheat temperature tends to decrease with decrease in load, means for controlling the firing of the furnaces including means for adjustably proportioning the total fuel fired between the two furnaces to increase the firing of said one unit relative to that of the other with decreasing load, and means independent of the firing rate operative to increase the heat content of the gases egressing from said one furnace with decrease in load.

18. The method of producing vapor comprising burning fuel at a first zone and creating a combustion gas stream and burning fuel at a second zone and creating another combustion gas stream, forcing the primary fluid in a continuous circuit including conveying it in confined paths disposed about said'one zone and thereafter in confined paths disposed about said other zone, conveying said primary fluid thereafter in serial heat exchange relation with the combustion gas streamcreated in each of said zones and thereby heating said primary fluid to its desired temperature, utilizing a portion of the energy of said primary fluid and thereafter reheating the fluid by directing it in heat exchange relation primarily with one of said gas streams, with this reheating being at a location such that the reheat temperature tends to drop with decrease in load, as the vapor demand decreases from a predetermined value regulating the temperature and pressure of the primary fiuid issuing from said continuous circuit including controlling the total fuel burned in the two zones, and regulating the reheating of said fluid in cluding, of the total fuel burned, increasing the proportion burned'in the zone creating said one gas stream, and, independent of the firing rate, increasing the heat content of the combustion gases of said one stream.

19. The method of claim 18 wherein the heat content of the combustion gases of said one stream is increased by varying the zone of combustion at which said one stream is created with relation to said paths disposed about the zone of combustion.

20. The method of claim 18 wherein the heat content of the gases of said one stream is varied by introducing into the combustion zone gases which have imparted heat to the fluid being reheated.

References Cited in the file of this patent UNITED STATES PATENTS 2,685,279 Caracristi Aug. 3, 1954 2,840,054 Rowand June 24, 1958 2,863,424 Koch Dec. 9, 1959 2,876,748 Nelken Mar. 10, 1959 2,896,592 Heller et al. July 28, 1959 2,918,909 Nickel Dec. 29, 1959 2,952,975 Braddy. Sept. 20, 1960 2,959,014 Artsay Nov. 8, 1960 2,984,984 Dickey May 23, 1961 3,017,870 Profos Jan. 23, 1962 3,038,453 Armacost June 12, 1962 FOREIGN PATENTS 323,723 Switzerland Sept. 30, 1957 831,175 Great Britain Mar. 23, 1960 971,815 Germany Apr. 2, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 3, 135 245 June 2,, 1964 Elno M. Powell et. al,

v It is hereby'certifiedthat error appears in the above numbered patent requiring con ection and that the said Letters Patent should read as corrected below, v

Column 3". line 28 for "temperature" read temperatures line 29,: gfor- "maintain" read maintains column 4 line l9 "then expanded read then again expand-ed line 22,, for "oondu'i-t'ffi'f 'f" read ;---"--conduit- 67 line 24"fo'r" reh'e-ataers 58 and 60"" read Peheats. 58 and 60 oolumn 6, line '45 for "so that to be? read so as to be column 7 line 1 for "is" read- -as',-----; column 8 line 45 for "directing the" read directing generally the column 12 line 5 for "related? roadie- 1;; -rehe ated I Signed and sealed this 17th day of November 1964,

Attest:

ERNEST w.. SWiDER a EDWARD J. BRENNER- Altosting Officer Commissioner of Patents 

17. A ONCE-THROUGH FLOW VAPOR GENERATOR OPERATING ON THE REHEAT CYCLE AND INCLUDING A PAIR OF UNITS EACH OF WHICH HAS A FURNACE AND A GAS PASS EXTENDING THEREFROM; MEANS FIRING EACH OF THE FURNACES, THE THROUGH-FLOW CIRCUIT INCLUDING A PLURALITY OF HEAT EXCHANGE SECTIONS CONNECTED IN SERIES FLOW RELATION WITH REGARD TO THE THROUGH-FLOW SO THAT GENERALLY THE ENTIRE THROUGH-FLOW OF THE GENERATOR PASSES THERETHROUGH AND EACH OF WHICH IS ASSOCIATED WITH AND RECEIVES HEAT FROM ONLY ONE OF SAID UNITS, THE WALLS OF EACH OF THE FURNACES BEING LINED WITH HEAT EXCHANGE TUBES WITH THE WALL TUBES OF EACH FURNACE COLLECTIVELY FORMING ONE OF SAID HEAT EXCHANGE SECTIONS AND WITH OTHERS OF SAID HEAT EXCHANGE SECTIONS BEING COMPRISED OF TUBULAR ELEMENTS DISPOSED IN THE GAS STREAM ISSUING FROM THE RESPECTIVE FURNACES AND WITH AT LEAST SOME OF THESE HEAT EXCHANGE SECTIONS BEING DOWNSTREAM WITH REGARD TO THE THROUGH-FLOW OF THE FURNACE WALL TUBE HEAT EXCHANGE SECTIONS AND WITH EACH OF THESE DOWNSTREAM HEAT EXCHANGE 